The well-known robot type named SCARA robot is a serial kinematic manipulator primarily used for moving and rotating objects without changing the inclination of the objects. The manipulator comprises kinematic links coupled in series. These robots normally have four degrees of freedom in the x-, y-, z-directions and φz (rotation of the object about an axis parallel to the z-axis). For manipulating the object in the xy-plane, two arms coupled in series and working in the xy-plane are used. In order to achieve a movement in the z-direction a linear movement device is used. This device is arranged either after the arms coupled in series or before the arms coupled in series. In the first case the arms coupled in series must move the drive assembly for the z-movement and in the latter case the drive assembly for the z-movement must move the arms coupled in series. The drive assembly for the φz-movement will always be located at the extreme end of the kinematic chain of the robot.
Several of the properties concerning the SCARA-robot are improved with a robot, which manipulates an object through working in parallel, i.e. a parallel kinematic manipulator, PKM. According to the statements above, a serial kinematic robot comprises a large mass and thus becomes compliant with low mechanical natural frequencies, the accuracy is limited and large motor torques are required for accomplishing high acceleration, jerk and speed movements possible.
A parallel kinematic robot is a design offering a high degree of load capacity, high stiffness, high natural frequencies and low weight. Three arms working in parallel are required to obtain manipulation of a platform in three degrees of freedom, i.e. the x, y and z-directions in a Cartesian system of coordinates. Six arms working in parallel are required to obtain manipulation of a platform in all six degrees of freedom, i.e. the x, y, z directions and the rotation angle/inclination of an object arranged on the platform.
Ideally, an object ought to be manipulated by a total of six separate links, which transfer only compressive and tensile forces to the manipulated object to obtain a stiff and accurate manipulation. Generally, the PKM comprises three up to six first arm parts. As an example, a manipulator with four arms designed for four degrees of freedom has second arm parts sharing the six separate links. This is only possible with certain combinations of the links, as for example, 2/2/1/1 or 3/1/1/1. 2/2/1/1 means that two supporting first arm parts are connected to the respective second arm part, which comprises two links and another two supporting first arm parts are connected to the respective second arm part, which comprises a single link.
A known manipulator is manipulating a platform, which remains with unchanged inclination in the whole working area. The robot has three supporting first arm parts, each connected to a second arm part, in kinematic parallelism. From this robot, it is known to arrange a total of six links optionally distributed on three first arm parts according to the combinations 2/2/2 or 3/2/1.
A known device for relative movement of a first element in relation to a second element according to the combination 2/2/2 is disclosed in the international application WO 99/58301. The three arms, each comprises a supporting first arm part connected to a second arm part, which includes a link arrangement. The first element is described as stationary and the second element is manipulated in the x-, y- and z-direction by driving means. Each link arrangement is connected to a supporting first arm part and to the second element, respectively, by means of joints of 2 or 3 degrees of freedom. Each driving means comprises a stationary portion and a rotating portion, where the stationary portion is included in the first, stationary element. Using the reference numbers in the document, each driving means has its rotating portion connected to the first arm parts 6, 7 and 8. The driving means 3 is pivoting the first arm part 6 and the driving means 4 is pivoting the first arm part 7 about the same geometrical axis 37. The third driving means 5 is pivoting the first arm part 8 about a geometrical axis 38, which is non-parallel to the pivoting axis 37. The third driving means 5 implies that upon pivoting of the supporting arm part 7 by means of the driving means 4 also the supporting arm part 8 will accompany as a consequence of the fact that an axis 53 and also a gear wheel 10 will accompany the pivoting movement. Thus, the driving means 4 and 5 must accelerate more and are more heavily loaded compared with the driving means 3. Consequently, this manipulator design necessitates three different driving means designs with three different drive dimensions. This makes the design more complicated and the manipulator relatively expensive to process. Another consequence is that the first driving means carries the highest moment of inertia and there will be an uneven distribution of the moment of inertia in the manipulator. Moreover, the mechanical natural frequencies will be lower because of the extra mass that axis 2 has to rotate, which gives a less accurate control at higher motion frequencies.
A device for relative movement of a first and a second element according to the second combination 3/2/1 is disclosed in the international application WO 97/33726. The device comprises a manipulator including three arms each arranged to connect a stationary and a movable platform. Each arm comprises a supporting first arm part and a second arm part connected to each other, where respective second arm part comprises a link arrangement. Three actuators are fixed to the stationary platform and actuate one first arm part each. A first supporting arm part is connected to a second arm part linkage arrangement comprising three links in parallel. Another first arm part is connected to a double link arrangement and still another first arm part is connected to a single link, where all links are connected to the movable platform.
The document U.S. Pat. No. 5,539,291 shows a parallel kinematic manipulator. A stand sustains a biaxial controllable supporting arm part. This arm part supports, in its turn, a second arm part, which sustains a movable object. A first and a third supporting arm pivoting around a common pivot axle are connected to the movable object via outer arms comprising belts with the function of a combination between an arm part and a four linkage. The outer arms and the second supporting arm are arranged to transmit compressive and tensile forces as well as torsion moments. The result is a relatively bulky design of a manipulator with a limited operating volume. Moreover, the shown manipulator comprises less stiffness, lower accuracy and much lower mechanical natural frequencies when compared with a manipulator comprising arm parts transmitting only compressive and tensile forces.
A robot is operating within a volume needed for the application, which is referred to as the operating volume in the following. Furthermore, the volume outside the operating volume, which a manipulator needs for its own purpose, is referred to as the unused operating volume. Prior art includes a manipulator, which has a voluminous and expensive design with a limited operating volume (FIG. 16 in the prior art). For certain robot applications, it is important due to enormously high initial costs to make a PKM with a small unused operating volume in relation to the operating volume and which can work close to each other.
According to the conditions mentioned above, there is a need for an industrial robot with high accuracy and stiffness. Further, there is a need for a robot with an improved course of dynamic forces and simultaneously an increased working volume in relation to the unused manipulator volume. Further, there is a need for a robot, which has the characters of rapidness and an exact movement. Additionally, there is a need for a robot design which makes the robots work close to each other.
The known industrial robots comprising a parallel kinematic manipulator do not satisfy this need.